If you want slow backups you can have the TC in another room connected by wireless.
But you need to realise you cannot connect to the TC in any way directly.. so even though it is sitting in room Y instead of room X it is now as useless as can be.. ethernet is off.. wireless does nothing but connect it to the main router..
So say I sit next to the TC. all communication to the TC goes.. computer---wifi---main router--wifi--TC.
You plug in an ethernet it is dead.. so the TC is not achieving anything in being in room Y.. you would do much better having it in room X plugged directly into the router. It will be 2x or better faster.. ethernet will still work.. 5ghz wireless will still work.. overall huge improvement.. Otherwise having spent all that money for AC .. you have rendered it useless.. AC will no longer transmit at all.
If it is necessary to have the TC in room Y with router in room X then solutions are in order best to worse.
1. Use ethernet.
2. Use EOP (homeplug) adapters. These do not always work but when they do can work well. If the store you buy from has return policy buy and try.. return if it fails.
3. Using the Extreme plugged into the router and wireless extend that to the TC is hugely better than joining the Fritzyboy wireless. This method leaves the TC working as AP still and switch.
4. Join wireless network. As posted above the absolute worst option.